When a porous substrate needs to be treated by etching or modification of a surface thereof, damage of the substrate often occurs. This is particularly true with plasma mediated treatments. This is presumably caused by active plasma radicals penetrating deeply into the porous substrate and reacting therewith, thereby changing its composition and its porosity. Both oxidative and reductive plasmas have such detrimental effects. These problems for instance occur in the field of microelectronics during integration of low-k dielectrics.
Low-k dielectrics (materials having a dielectric constant lower than the dielectric constant of SiO2, i.e. lower than 4.0) are necessary to decrease capacitance between nearby conductive portions of high density integrated circuits and thereby avoiding loss of speed and cross-talk. In order to decrease the dielectric constant of low-k dielectrics as much as possible, low-k dielectrics are made porous. Thereby, the dielectric constant can be lowered down to about 2.0 or even less. Integrated circuit fabrication processes on dielectrics involve plasma etching and expose therefore the dielectrics to the damages mentioned above.
US2005/0148202 describes a method for sealing or protecting porous materials used in semiconductor fabrication. It describes sealing the pores of a porous material by applying a mixture of a polymer compound and an organic solvent. The sealing layer thus formed is further dried resulting in evaporation of organic solvents and volatile constituents (if any), and securing of the polymer compound on the surface as a sealing material. Such a sealing method has however several drawbacks. The long contact time between a hot solvent and the substrate makes dissolution or damage of the substrate possible.
Furthermore, the method is rather complicated, tedious and labour-intensive since it involves synthesising a particular polymer having well defined end-groups, preparing a particular polymer solution, applying it homogeneously on the substrate (this implies good wettability and elaborated application techniques), and heating the solution to evaporate solvent and/or dry the polymer. Furthermore, polymer deposition typically generates stresses in porous substrates. Also, the polymer being retained in the pores in the final product, it potentially influences the properties of the dielectrics making them harder to control. It also raises the question of mechanical stability when there is a mismatch between the thermal coefficient expansion of the polymer and of the porous material. Last but not least, polymers tend to have difficulties completely filling pores and/or entering the smallest pores, resulting in a filling which is not optimally dense (see FIG. 7 (P)).
Dubois et al (Adv. Mater. 2011, 23, 25, 2828-2832) discloses a similar method for sealing porous low-k dielectrics with an organic polymer. The polymer is degraded by thermal treatment once the etching and other processing steps are performed. This presumably permits the patterned dielectric in the final structure to have comparable properties to its pristine equivalent. However, removing a polymer by thermal means implies degrading it and thereby the possibility of leaving polymer residues in the pores. Also, it is energy intensive. Furthermore, the other drawbacks proper to the use of polymers remain as mentioned above for US2005/0148202.
EP2595182 discloses a method for treating a surface of a porous material in an environment, the method comprising the steps of setting the temperature of the surface to a value T2 and setting the pressure of the environment to a value P1, contacting the surface with a fluid having a solidifying temperature at the pressure value P1 above the value T2 and having a vaporizing temperature at the pressure value P1 below 80° C., thereby solidifying the fluid in pores of the material, thereby sealing the pores, treating the surface, wherein the treatment is preferably an etching or a modification of the surface, and setting the temperature of the surface to a value T3 and setting the pressure of the environment to a value P2 in such a way as to vaporize the fluid.
Although this method is effective at preventing much damage to porous substrates, some damages still occurs.